Modern automobiles have an ever-increasing need for electrical power, due in part to the continual addition of electrical and electronic apparatus of various types for improving engine performance, steering and other functions of the automobile, as well as air-conditioning units, stereo systems and the like. This has placed an increasing demand on the conventional automobile generating system, requiring higher power alternators and generators and consequent higher drain on the engine output, leaving less horsepower available for propelling the automobile.
There is known a piezoelectric power system for converting waste acoustical energy from an aircraft jet engine exhaust into electrical energy, U.S. Pat. No. 3,239,678. There piezoelectrical elements are mounted to be driven in a simple beam mode by the jet exhaust, where the major portion of the acoustic energy occurs in the frequency range between the rotation frequency of the engine and the blade frequency of the turbines, typically several hundred to several thousand Hz. In contrast, the acoustic energy in the exhaust systems of conventional reciprocating piston engines is in the range of the rotation to the piston frequency, namely 50-600 Hz. Unfortunately, the piezoelectric transducers disclosed for use at the higher frequency of jet engine exhaust would be much too large to be practical for resonant operation at frequencies as low as 50-600 Hz. For this reason, piezoelectric generators are not currently in favor for use with automotive engines, even though the engines have become smaller and the power consumed by the electric generator, typically several horsepower, has come to represent an increasing and significant portion of the engine's available power.